COMSTAR COMETH: Introduction & iSCSI Target for COMSTAR

09 Sep '08 - 09:10 by benr

The OpenSolaris COMSTAR project is one of the cornerstones of Sun's Open Storage initiative. Quite simply, it uniquely separates and abstracts back-end storage from the means by which you allow access to it. To be more specific, through COMSTAR's SMTF I can allocate some raw disk and then provide it to my SAN through an FC Target or iSCSI Target or SAS Target or you name it. Just think about this... you can now, with 100% open source software, turn a cheap white-box PC with a bunch of cheap SATA disks and a Fibre Channel HBA off eBay into what appears to a SAN to be an enterprise grade Fibre Channel array!

COMSTAR is short for "Common Multiprotocol SCSI Target". It's core is the "SCSI Target Mode Framework" (STMF). The STMF acts like a hub into which you plug Logical Unit Providers (LU... disk, allocated space on some device, tape, etc) and Port Providers (Target Implementations; think "port" as in Server Daemon). This abstraction is interesting because it means I could potentially take one allocation of disk and provide it as BOTH an iSCSI Target AND a Fibre Channel LUN!

Now, I'll be honest, I think the concept of turning a commodity server into a Fibre Channel array is really nifty and all, but I'm not gonna do it... I don't care. But iSCSI I most certainly do care about. It came to my attention this morning that its already available and been in development for some time, hiding as the iSCSI Extensions for Remote DMA (iSER) Project. This project is not just extentions... it is the COMSTAR iSCSI Port Provider itself, and it includes iSER support. If you've been waiting for the new iSCSI Target Implementation that will utilize COMSTAR, this is it!

Lets dive into this on a high level; in future blog entries I'll dig into it much more deeply.

To get started, go to the poorly named iSER Project and download one of the binary bundles. You'll need to be running snv_94 or newer... if your not, BFU to it now. The binary bundle contains 4 packages, install them all.

Once you've got things installed, engage COMSTAR (the stmf service) and enable the iSCSI Target Port Provider. If you have been using the old iSCSI Target (iscsitadm, and co.) disable it first.

smtfadm: The STMF management CLI; it is the most central of all COMSTAR related commands which maps Port Provider resources (targets) to Logical Unit (LU) Providers (storage)

sbdadm: The SCSI Block Disk (LU Provider) CLI

Creating iSCSI Targets with COMSTAR is strange at first because unlike the current Solaris implementation ("iscsitadm") you do not specify what storage to use when you create an iSCSI target. To review, with the current target you would use a command like this:

What this iSCSI target will point to is handled by STMF, which will map this target to an actual resource. So the next step is to create the actual Logical Unit resource using "sbdadm" for SCSI block disk. I'll create a ZFS Volume for this purpose:

So now we have our Logical Unit (LU) to plug into our target. We now need to use "stmfadm" to map them together into a "view". Storage administrators will no doubt be familiar with the task of mapping Volumes or Disks to Targets, this is the same task. Views are linked to a LU, thus a view can only refer to a single LU, but a single LU can have multiple views (more about this later).

Views or Mapping, comes in two forms: Simple and Selective. In simple mode you allow anything to map to the LU... in selective you actually create "host groups" (which initiators/clients can see the LU), target groups (which targets, or port providers, will be able to provide access to the LU), etc. To keep our tutorial here short I'll opt for the "Simple Mapping" method.

In full disclosure, while the device is available for some reason I can't partition it... but I'll work that out shortly.

COMSTAR is an amazing thing and so far the iSCSI Target looks good. iSNS support, TPG support, RADIUS/CHAP support, and of course iSER support are already there and humming along and I can't wait to test these, especially iSNS integration.

Please note, this is heading toward but is not yet integrated. ZFS "shareiscsi=on" support is not currently available but will when the time comes.

A very warm thank you to the team who's been working on this.

- - C O M M E N T S - -

you can now, with 100% open source software, turn a cheap white-box PC with a bunch of cheap SATA disks and a Fibre Channel HBA off eBay into what appears to a SAN to be an enterprise grade Fibre Channel array!

Ben said: “Just think about this… you can now, with 100% open source software, turn a cheap white-box PC with a bunch of cheap SATA disks and a Fibre Channel HBA off eBay into what appears to a SAN to be an enterprise grade Fibre Channel array!”

You go do that, then discover that you have many problems with the motherboard and CPU (sensors don’t work, powerscaling does not work, SATA works only in legacy IDE mode, some SATA devices are not recognized at all), then join the forums and list your problem and get shouted down as a Linux zealot.

We’ve been trialling COMSTAR on x4500s as a FC target for the last few months and we’ve been getting comparable performance (better for some workloads, worse for some) to our “real” SAN equipment.

I had secretly expected that the iSCSI port provider would allow to export the same LUs over iSCSI as well and I’m very pleased to find out that they do. Add CIFS, NFS and the new FCoE provider ([[http://opensolaris.org/os/project/fcoe]]) – nice, but not something I’d necessarily want to be rushing into production OpenSolaris has everyone’s bases covered.

Florin: This isn’t a windows or linux blog, don’t be so whiney and naive. Try wrapping your brain around the content at hand and maybe smile once awhile.

benr. - 09 September '08 - 18:11

Ben, I was only trying to temper down your enthusiasm about throwing Solaris on any random collection of parts acquired from eBay or newegg. And that wet blanket comes from the Solaris “old hat, crusty admin, we know better than the hippie OS” community…

Took me about 5 min since the download to prepare a 100GB target on a snv_97 box over a 240GB striped pool. W2K3 iSCSI initiator had no problems partitioning and formatting the drive. Using identical striped disks physically connected on the W2K3 server, HD-Tach reports 64MB/s, on the iSCSI target – about 45MB/s (1 gb/s link). So performance is quite reasonable as well.

Thanks for pointing out to me this project, it has slipped me (I usually make iSCSI targets on old machines using NetBSD).

Ben, do you have any comments on how COMSTAR guarantees synchronous writes? ZIL can certainly kill performance. I am really wondering what kind of numbers people can get with synchronous writes with ZFS and COMSTAR.

Leonid - 12 September '08 - 04:55

Leonid: Sadly I don’t have a proper benchmarking environment currently in place, I’m currently in the processes of finally dumping my crappy Athlon64 system (the Tyan motherboard has been nothing but problems) and rebuilding it with Intel Quad Core so that I can benchmark against my primary box which is an Ultra 40.

When I get my new rig in place I plan to do some benchmarking of the old iSCSI Target vs the COMSTAR target under various scenarios, as well as FCoE benching, etc. Those types of tests are worthless between virtual instances, imho.

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